1. Field of the Invention
The present invention generally relates to a CMOS power sensor, and more particularly to a CMOS power sensor having a current coil which is fabricated during the process steps of forming gold bumps of a CMOS device.
2. Description of the Related Art
A Hall device is a voltage generator based on the famous Hall effect to give an output voltage proportional to the magnitude of an external magnetic field. The Hall effect was discovered by E. H. Hall in 1879. When an electric current flows through a conducting material in a magnetic field, the charge carriers of the electric current will be pushed to one side of the conducting material by the transverse force induced by the magnetic field. Hall found that a measurable voltage would be induced between the two sides of the conducting material.
The conventional Hall devices are generally applied to detect the magnitude of an external magnetic fields. For example, the Hall device can be used in the computer keyboards, where pushing a key moves a permanent magnet toward the Hall device. The Hall device detects the change of the magnetic flex of the permanent magnet, and induces a Hall voltage which can be detected by the computers.
In addition, when located near a current coil, the Hall device will be subjected to the magnetic field which is generated proportional to the current magnitude flowing in the current coil. Therefore, the Hall voltage generated by the Hall device will be in proportion to the magnitude of the current flow in the current coil. For this reason, the Hall device can also be applied to detect the current magnitude of an input voltage source, and also to detect the power magnitude of an input voltage source.
Recently, research was dedicated to developing solid state micro-sensors based on the CMOS process for the detection of the current magnitude or power magnitude of an input voltage source. Hong-Ming Yang et al. developed a MOS magnetic current sensor based on standard CMOS process, which was published at ELECTRONICS LETTERS, 27th Match 1997, Vol. 33 No. 7. In that letter, they reported a current sensor based on standard CMOS processing. The current sensor has a high sensitivity owing to its current coil, fabricated on the second metal layer, being very close to the magnetic sensing MOS device underneath. The current coil has thus the same material with the second metal layer, i.e. aluminum (Al).
However, one disadvantage of using aluminum to form the current coil is that aluminum has a relatively high electric resistance. An Al-made current coil will result in a relatively large voltage drop, and will thus change many electric properties of MOS devices. This problem is particularly serious in low-voltage devices, where even a small voltage drop can have a profound effect on the devices.
Therefore, it is important for the IC-related industries to invent a new type of current sensor or power sensor, which contains a current coil made of a material having relatively low electric resistance, in order to achieve a relatively low voltage drop at the current coil and minimize the effects on the electric properties of the MOS devices.